Some busy double track mains still have the occasional short section of single track. For example, I understand that the bnsf transcon still has a couple of short single track sections at some river bridges. So, if this ever happens, maybe it is not 100 percent 2 tracks. Unless government puts up big $ to do this, I don’t see it unless csx has plans for sneaky big increases in tonnage. With adequate, lengthy sidings, quite a bit of volume can move on single track ctc. Perhaps more sidings are added first.